Optical waveguides capable of transmitting an optical signal with only one direction of polarization are desirable for use with integrated optical devices. However, it is well known that even the so-called "single mode" fibers do not maintain linear polarization over extended distances. As a result, the polarization of a signal received at the outpt end of a fiber is uncertain and, indeed, changes with time. To accommodate such signals, efforts have been made to devise polarization-independent optical devices. (See, for example, U.S. patent application Ser. No. 14,832 by R. C. Alferness, filed Jan. 24, 1980, now U.S. Pat. No. 4,273,411.) However, there is a class of interferometric signal processing techniques for which the polarization of the received signal must be along a specified direction. Heterodyne detection and fiber interferometers are two such examples.
One solution to this problem is to use specially prepared fibers which employ stress-induced birefringence to maintain linear polarization. (See U.S. Pat. No. 4,179,189.) However, these fibers exhibit rather high losses and, to date, only short lengths have been made.